Carb problem

I have a Rochester AA carb on my 1950 Olds (Backdraft style) and have the following problem. If I start the car everyday it start fine. If I let the car sit for more than 2 days it will not start at all without a lot of pedal pumping.

It seems to me that the carb must be slowly leaking gas and after a few days the bowl must be empty. I had a hard time starting it last night and this morning there is a noticeable gas smell coming from the engine. No external leaks though. Also, I can see fuel in the fuel pump gas bowl and it seems to pump fine.

I had the carb out recently, filled the bowl and let it sit on the workbench overnight but did not see any leak. All the symptoms though make it seem like it is out of gas when it won't start as it will crank but not give even a hint of trying to fire. When it does start it does not bellow out any smoke or run like it is flooded. I know there can be issues of boiling the gas out of the bowl but since the car is still being restored and not on the road I generally only run it for a few minutes.

Any suggestions are welcome.

My first thought is high-volatile ethanol fuel, especially if the fuel is more than a few weeks old.

If it's been as warm in NJ the last few days as here and the fuel is still winter-blend gasoline or one of EPA's region-specific blends, wouldn't be unusual for it to evaporate out of the bowl after a couple of days.

Such is the lot of carbureted people in a fuel-injected world.

Are there any fuel additives I can try that will counteract this issue?

Are there any fuel additives I can try that will counteract this issue?

Yes, at least to some degree. It is LUCAS Ethanol treatment. LUCAS makes a number of fuel treatment products just make sure to get the one with Ethanol prominently on the label.

If that doesn't solve the problem look to issues with the fuel pump.

The Lucas "green stuff" which Jim mentioned. Many of the other Lucas fuel additives I've found are not "green".

What does the engine's dip stick look/smell like??? If it's hard to start, initially, but doesn't act like it's flooded, then the fuel has to be "draining" somewhere, which is usually into the intake manifold, then the cylinders, and then the oil pan.

Before you start it the first time, have you checked the accelerator pump's output? I presume it has some sort of accelerator pump? If the pump shot is good, before the engine is attempted to be started, then there should be some fuel left in the float bowl . . . the accel pump usually feeds from the lowest part of the float bowl.

What do the spark plugs look like? Fluffy carbon-coated insulator? Clean? (and this might sound a little flaky) Is the electrical system grounded to the proper side of the battery? If a "positive ground" car is hooked up as a "negative ground" car, the ignition system's output will be a good bit lower than it should be.

What about the newness and cleanness of the ignition points?

Something tells me that this might not be a fuel issue, but an electrical issue . . . sometimes, fuel and electrical issues can act similarly.

NO implication that anything was intentionally done incorrectly, just some thoughts about what the ultimate issue might be.

Please keep us posted on your progress.

Just some thoughts . . .

NTX5467

Evaporation is a COMMON problem today:

THE CARBURETOR SHOP / Troubleshooting

Jon.

I have had success with adding 1 gallon of diesel to a tank of gas. (was suggested by someone on these forums when I had the same issue) Kinda returns todays gas back to its far oilier past! Slows down the evaporation and serves as a top cylinder lubricant. Of course this works as long as the issue isn't what was outlined by NTX5467 above.

Scot

This is really driving me crazy. I had the radiator out for a few weeks while I painted it so the car sat. Went to start it yesterday and it woud not fire. I have installed a 6 volt fuel pump to ensure I have pressure as well.

I pumped the pedal once to set the choke which is fully closed but not hard enough so I can't push it open. Cranked and no ignition. I tried cranking for about ten seconds without touching the gas pedal, stopped, pumped again in case the bowl had been empty and still no start. After more cranking I'm not sure if it has no gas or is flooded. The Rochester AA carb is back draft so it is impossible to see if the accelerator pumps work although every time I had the carb off and tested them they sprayed a nice stream. I gave it a shot of starter fluid and the engine fired and went on fast idle like you would expect. I did not notice any excessive stumbling or smoke like it was flooded.

To answer a few of the questions above...

I added Stabile to the gas

Can't see in the carb if accel pumps are working. Backdraft carb (Original). I have cleaned it all, replaced the needle and seat as well as confirmed the float does not have any pin holes. Sat the bowl on my workbench full of gas and after 24 hours did not see the level go down.

Spark plugs looked dry last time I had them out. Not oily or wet. No clouds of blue or grey smoke when it starts.

Car is negative ground and I have a hefty ground strap from the block directly to the negative side of the battery. No ground strap from block to body though.

Points were new as part of the rebuild. 0 miles on the engine. I'll pull the cap and re-check the gap and ensure they are not corroded.

Strangest part of all is that when the body was off the car it started every time.

I would seriously consider putting a different carb on it. Does anyone know if any after market would fit? I really don't want to change the manifold.

Might it be possible to get a 6" diameter cosmetics mirror to place behind the carb to see what might be happening in there? Just a thought. Might need an aux light source back there, too.

When you used the starting fluid, it started, and went to fast idle . . . what happened after it got warmed-up enough to go to "hot, base idle"? After some run time, would it restart easily?

We know that negative ground has a hotter spark than positive ground ever can. What's the gap on the spark plugs? Just curious. What did they look like after you got the engine to run?

Sounds like most everything is working as it's supposed to, but there's still "something" somewhere, it seems.

Please keep us posted,

NTX5467

The light and mirror trick won't work as these carbs are an insert that fits into a larger housing. You can actually leave the housing connected to the manifold, throttle linkage and choke and remove the internal piece with jets, float, accel pump... to work on it. Neat concept actually. You would need to go pretty deep into the carb and make a 90 degree bend half way in. to see anything

After the car fired up it went on fast idle and sounded great. Kicked it down and still fine. After it warmed it fell to normal idle and till everything was fine.

24 hours later I pumped it once and it fired right up. The common thread with this is having it sit for any period of time. I will wait 48 hours and see how that works. I don't smell any gas in the oil or at least the oil seems fine to me. I would think if the bowl was leaking slowly that it would just evaporate in the intake manifold. Still, it took forever to start. A lot more than I think it should take to get gas in the bowl. At one point I had the float adjusted incorrectly so it never fully closed and a strong stream of gas shot out the carb inlet which leads me to believe I am getting plenty from the pump. As a backup plan I added the electric pump between the tank and mechanical pump. Fuel filter is new and clean. All of that plus the fact that when it starts it runs just fine leads me away from a fuel restriction issue.

I was wondering if this is just choke related. When cold I have the auto choke set so it is fully closed and requires mild pressure to open by hand. Could it be I need more pressure on the choke plate? Maybe it's just too lean to fire when cold if the choke plate is opening from the vacuum created during cranking? Again, I can't see what is happening as you can't see the carb inlet while starting as it is too close to the firewall. A mirror setup for that might work. I have the Rochester carb manual and have set it up per the instructions but regarding the choke it just says to adjust it to be closed. No reference to any other fine tuning. It has been about 20 years since I messed around with carbs and chokes. I understand the principals but lacking on recent, practical experience. Maybe I have some fuel evaporation + bowl leakage + bad choke settings.

I'll try a different additive as well and just keep extending the times between starts to see how many days it take to be a problem. Maybe that will tell me something

Any Rochester AA carb experts out there?

Sounds to me like the bowl is dry (evaporation). I know on my '62, if I let it sit for a period of time, it takes quite a bit of cranking and pumping to get her to fire. If I keep after her every day, she'll fire off just fine on the first try. It's today's fuel - it evaporates too quickly.

Paul

The next time you let it sit that long turn on the key and let the electric fuel pump fill the carb and before you do anything get out and manually pump the linkage to see if it is squirting fuel (actual fuel not water) into the intake. If it is then you have another problem, if it isnt that is the problem, the electric fuel pump isnt pushing past the manual pump. VERY common problem, especially with manual fuel pumps with sediment bowls on them, the electric pump pushes the water and whatever sediment into the carb and cant get fuel in there till the water is gone. Try using just the electric fuel pump bypassing the manual pump and try it.

Thanks for all the replies. Two issues. Fuel evaporation as many of you have said combined with a loose heat tube on the choke. Starts up great every time now. I have a 6 volt pump and was thinking about using it only when needed by having it bypass the mechanical fuel pump with T connection at the mechanical pump inlet and check valve at the outlet. This way I can turn it on if the car has been sitting to get it started and turn it off after the car starts up. Any else done this?

You dont need to bypass the manual pump. Mount it before the manual pump close to the tank and just switch it on and off. It wont hurt the manual pump diaphragm at all.

You need to mount it as low as possible and as close as possible to the tank so it dont starve for fuel when your level gets low. Make sure you put a filter before the electric fuel pump, the least little trash will lock it up.

My reasoning for bypassing the pump is that the electric does not seem to be able to push through the mechanical. My mechanical works fine but when the carb bowl is dry it does not matter how long I let the electric pump run, it still takes a lot of cranking before it will start.

The trick I learned re: this problem is to manually fill the fuel bowl thru the fuel bowl vent(s). To do so I use a clear plastic ketchup squeeze bottle. Hey, not elegant, but it works as the car fires right up!

I wish it was that easy. The carb inlet faces the firewall so I would need to remove the air cleaner and then squirt the gas horizontally into the vent. I can see now why they only used this style carb for a year or two.

My reasoning for bypassing the pump is that the electric does not seem to be able to push through the mechanical. My mechanical works fine but when the carb bowl is dry it does not matter how long I let the electric pump run, it still takes a lot of cranking before it will start.

Bypassing at the fuel pump would be a good option but i dont know any kind of check valve that can be used with fuel. It would have to be 1 with brass ball and seat. blow thru your fuel pump in both directions and see if it will flow thru backwards. if it wont there is your built in check valve. The mini electric fuel pump i had before would only flow 1 direction.

Your problem is a common occurence for 1949-1950 Oldsmobile owners equipped with AA carb. The AA carb is simply just another zinc die-cast carburetor in use in 2012 with evolving gasoline formulation, with most pump gas now formulated for fuel injected vehicles with electric primary and/or booster fuel pump designs.

The accelerator pump is a center actuated diaphragm design (reminding one of later Holley design) located at the bottom of the carb body assembly. This location means any water in the fuel whatsoever will come to rest within the pump cavity, causing even short storage failure of the pump. While this might make the alchohol content in gasoline at first glance appear to be a positive in displacing water in the fuel, unless your carb has been rebuilt with the very latest materials the alchohol attacks the rubber acc pump material most older carbs and kits contain. Further, the level of alchohol attacks the zinc parts even with good plating intact, creating a white residue which is very difficult to completely remove during service. The kit offered by Daytona(?) has the newest materials with proven success with cheap gas, and yields the best result for drivers of these cars. The real solution for you is a rebuild with one of these kits, a rebuild of your mech fuel pump with same modern materials, and a electric booster pump pushing through your existing mech fuel pump (with positively no reason to bypass existing mech fuel pump), hidden aft near the fuel tank. The final important piece is the use of only top tier gasoline, which as a Oldsmobile Aurora owner I discovered (too late) after GM directives mandated for use after the beginning of gas formulation change in 2005:

Top Tier Gasoline

In our area only Shell has top tier, and only in their premium with V-power. It has the detergent and other additive packages benefiting our use of this fuel with carburetors and mechanical fuel pumps. It smells and looks like gasoline, not sour solvent. Our local group of Oldsmobile guys all run it now, and have seen big mileage improvements, easier hot starting, eliminated corrosion problems, much better looking spark plugs, and better, smoother power and operation especially on the open road. Even on the hottest day this summer, my '52 lights right off, no more endless hot start cranking and swearing, the hot day cranking adding a exponent of use and wear to starter, battery, cables and charging system afterwards. About $5 more per tankful, but radically increased mileage per tank is netted so no real cash loss. Top tier gas also has a useful extended storage life, even with out adding stabilizer.

One other thing, engines running the AA start and drive best with the choke set to normally close, even in hot weather. If you don't set it to close, you have to pump your ass off to start and keep it running up to operating temp. We know the AA pump is barely adequate even when 100%, so set the choke to barely but definately close on a warm day, and as you indicated already with heat pipe in good order so it may open.

The 'go to' guy for AA is Franklin Graf in Seville Ohio. He rebuilds AA's with the proper kit, tests the rebuild on one of his cars before shipping. Contact him here:

2012 Oldsmobile Car Show Event for 1949 through 1959 Olds Rocket Powered Engines vehicles auto automobile shows in Ohio

CW

Great write up "CW". I took my fuel pump a part as it developed a significant leak and found the diaphragm has become porous and is allowing fuel to leak past it and into the body of the pump which in turn pours out what seems to be a vent. For now I bypassed the pump and will look into a modern rebuild kit for it.

Regarding the choke... I have had a difficult time getting that to stay were I set it. One day I will pump the pedal and the choke would close, the next it seems to have no pressure on it. i adjust and a few days later it seems to close extremely hard and flood the engine. Recently I took the choke coil spring out and turned it around. Since then it seems to be fine.

As of today the car starts right up and cruises fine at highway speed. I plan on always using top tier gas but think I should also put an additive in it to reduce the effects on the rubber parts.